some exec/arguments issues

first, exec.command has signature with variadic arguments:

command(name string, args ...string) command

but user functions cannot have variadic arguments, nor there is spread operator. So, I cannot write function like:

func myexec(cmd, args) {
c := exec.command(cmd, args) // type error: expected a string (list given)
exit_code := try(c.run, 1)
if exit_code != 0 {
print("COMMAND FAILED:", cmd, args)
print("STDOUT:", c.stdout)
print("STDERR:", c.stderr)
}
}

second, it would be nice to print stacktrace when error has occured, because raw error message says nearly nothing
third, it seems shebang shenninghans for cli package doesn't work for me.
- first issue is that shebang passes arguments like so:
```
$SHEBANG $SCRIPT $ARGS
```
  so, when shebang is #!/usr/bin/env risor -- it gives
```
/usr/bin/env: ‘risor --’: No such file or directory
/usr/bin/env: use -[v]S to pass options in shebang lines
```
  If we fix shebang to #!/usr/bin/env -S risor --, following program is being executed:
```
/usr/bin/env -S risor -- script.risor $ARGS
```
  which in turn calls
```
risor -- script.risor $ARGS
```
  BUT, as I understood, risor for some reason requires script to be in form risor script.risor -- $ARGS so actual behavior now is open risor repl. Though, for now this can be sidestepped with following shebang:
```
#!/usr/bin/awk BEGIN{for (i=2; i<ARGC; i++) { subslice = subslice ARGV[i] " " }; system("risor "ARGV[1]" -- "subslice)}
```
- second issue is that when we finally were able to call risor correctly, os.args gives array of strings from $ARGS we passed along. BUT it should also include the script, such that os.args() == ["./script.rasor", $ARGS...]. Because of that, I cannot pass os.args() to cli.app.run() because the latter requires executable to be the first argument, which again can be sidestepped for now with .run(["vahui"] + os.args())
one more issue is with running exec: there is no way to catch exit code. The only way to not fail script and handle failed command for now goes like
```
c := exec.command(cmd, args)
exit_code := try(c.run, 1) // 1 is returned instead of actual non-zero code
if exit_code != 0 {
print("COMMAND FAILED:", cmd, args)
print("STDOUT:", c.stdout)
print("STDERR:", c.stderr)
}
```
such wrapper can be useful for e.g. printing failed command, printing it's stderr to get reason of failure, instead of just
```
exit status 1
```
which is not informative at all.

risor-io / risor

some exec/arguments issues #229